As is known, for such applications, for example in the automotive industry, it is required that the plastic materials to be sold be accompanied by a declaration certifying their characteristics, in particular, their temperature resistance under load, by providing their values of flexure and deformation under predetermined test conditions. In particular, numerous international standards exist which regulate these test conditions, such as the standards ISO 75 (part 1 and 2), ISO 306, ASTM D648 and ASTM D1525 respectively for the HDT tests (Heat Deflection Temperature) and the Vicat tests.
Specifically, in the HDT test, one measures the stress induced in a specimen subjected to a flexural load at 3 points. To this end, the specimen is subjected to a load of predetermined value, for example by way of a bar-shaped head; it is then heated gradually and in controlled manner (2° C. per minute) until reaching a predetermined deflection (0.25 mm for the ASTM test or a value between 0.32 and 0.36 mm for the ISO test). The temperature value at which this deflection is reached represents the HDT value.
In the Vicat test, or softening test, one measures the temperature at which a circular indenter with a cross section of 1 mm2, subjected to a predetermined load, penetrates by 1 mm into the specimen.
For the performance of such tests, as set forth by the standard, the specimen is first secured to a support and immersed in a liquid able to transfer heat under the indicated controlled conditions.
Hence, for some time, test machines have been available on the market having a tank filled with a heat-transfer fluid, typically silicone oil, which is heated and cooled by means of coils and fans so as to provide the required heating ramps of the test. In such machines, a specimen secured to a support is immersed in the tank of oil and subjected to the action of the load (in the HDT test) or the indenter (in the Vicat test). The deformation of the specimen is measured as a movement of a rod secured to the load or carrying the indenter; as soon as the rod moves by the value specified in the standard, the current temperature value of the heat transfer oil is acquired, representing the HDT value or the softening value, depending on the type of test. At the end of the test, the oil is cooled down so that another test can be performed on a different specimen.
In the testing machines of the type under consideration, therefore, the heat transfer oil undergoes many cycles of heating (up to 290° C.) and cooling which, over time, degrade its characteristics, especially its viscosity. Thus, after a certain time, the heat transfer oil deteriorates and needs to be replaced, in order to assure the proper heating of the specimen and therefore proper performance of the tests.
With the current testing machines, however, the determination of the time at which the heat transfer oil deteriorates is not possible with precision. In fact, tables provide the deterioration times of the heat transfer oil as a function of certain temperature values. But these do not take into account the variability of the temperature during the tests and do not provide degradation values at different temperatures. Consequently, such tables are not directly useful with the testing machines under consideration, given the above-described cycles of heating and cooling.